Democrats and some former members of the military reacted with anger and sadness to the dismissal of Gen. Charles Q. Brown Jr. as the chairman of the Joint Chiefs of Staff, arguing it was part of a political purge of military officers by President Trump.

On Friday evening, Mr. Trump announced he would replace General Brown with a little-known retired Air Force three-star general, Dan Caine. Mr. Trump and Defense Secretary Pete Hegseth have promised to fire “woke” officers and instead promote officers steeped in a “warrior culture.” Five other Pentagon officials were also fired that evening.

“Trump wants to make sure that the Joint Chiefs of Staff are 100 percent loyal to him,” Representative Adam Smith, Democrat of Washington State and the ranking member of the House Armed Services Committee, said in an interview. “I don’t think there is any question that is why he did it. There is no argument that General Brown isn’t an incredibly capable leader.”

General Brown’s dismissal took effect immediately. Pentagon officials said on Saturday that Adm. Christopher W. Grady, the vice chairman of the Joint Chiefs, is now acting chairman until the Senate confirms a permanent replacement.

Retired military officers argued that General Brown did not deserve to be fired and was the kind of war-fighting officer that President Trump said he wanted to lead the armed forces.

Mark Montgomery, a retired rear admiral and a senior fellow at the Foundation for the Defense of Democracies, said General Brown was a “proven war-fighter.”